Method and apparatus for making a blown article

ABSTRACT

THIS INVENTION COMPRISES THE PREPARATION OF A BIXIALLY ORIENTED, THERMOPLASTIC, CONTAINER-SHAPED ARTICLE BY INJECTION MOLDING A HOMOGENOUS MELT FOR FORM A HOLLOW BODY OR PARISON, COOLING THE BODY TO A TEMPERATURE RANGE WHEREIN THE THERMOPLASTIC CAN BE BIAXIALLY ORIENTED BY STRETCHING AND INFLATING, SAID RANGE BEING BELOW THE HOMOGENEOUS MELT TEMPERATURE, AND THEN STRETCHING THE COOLED BODY ALONG ITS LONGITUDINAL AXIS AND INFLATING IT RADIALLY WITH RESPECT TO SUCH LONGITUDINAL AXIS TO SO AS TO ONTAIN UNIFORM BIAXIAL ORIENTATION OF THE THERMOPLASTIC BODY. THE PREFERRED THERMOPLASTIC IS POLYVINYL CHLORIDE.

July 2, 1974 '7 J SCALQRA Re. 28,053

METHOD AND APPARATUS FOR MAKING A BLOWN ARTICLE Original Filed Aug. 11,1967 2 Sheets-Shoot. 1.

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METHOD AND APPARATUS FOR MAKING A BLOWN ARTICLE July 2, 1974 2Sheets-Shoot l Original Filed Aug. 11, 1967 INVENTOR- /4 4 64/0 BY MUnited States Patent ()fiice Reissued July 2, 1974 28,063 METHOD ANDAPPARATUS FOR MAKING A BLOWN ARTICLE Anthony J. Scalora, Toledo, Ohio,assignor to Owens-Illinois, Inc.

Original No. 3,470,282, dated Sept. 30, 1969, Ser. No. 659,968, Aug. 11,1967, which is a continuation-in-part of abandoned application Ser. No.275,735, Apr. 25, 1963. Application for reissue Sept. 27, 1971, Ser. No.

Int. Cl. 1329c 17/04; 329d 23/03 US. Cl. 26497 6 Claims Matter enclosedin heavy brackets appears in the original patent but forms no part ofthis reissue specification; matter printed in italics indicates theadditions made by reissue.

ABSTRACT OF THE DISCLOSURE Related application This application is are-issue of U.S. Pat. 3,470,282, issued Sept. 30, 1969, matured from US.Patent application Ser. No. 659,968, filed Aug. 11, 1967, which is acontinuation-in-part of copending US. Patent application Ser. No.275,735, filed Apr. 25, 1963, now abandoned.

The

This invention relates to a process for preparing a novel article ofmanufacture from thermoplastic material. More particularly thisinvention relates to a process for biaxially orienting thermoplasticmaterial during the formation of a blown or inflated container-likearticle such that the article has improved properties andcharacteristics, especially physical strength.

The phenomenon of biaxial orientation in thermoplastic material has longbeen recognized. It is known in the prior art that sheet and tubularmaterials, such as polyethylene, polypropylene and polystyrene undergosubstantial molecular transformation and exhibit marked increase inphysical properties if the sheet is stretched both longitudinally andtransversely after being formed. See, for example, US. Letters Patents3,231,642 and 3,283,317. One accepted explanation of the increase inphysical properties is that the stretching causes alignment of thepolymeric chain in the direction of stretching and that the alignedchain is better able to resist tensile strain and rupture relative tothe originally randomly oriented polymeric chain.

However, herebefore, the prior art has not effectively preparedcontainers or like shaped-articles which are biaxially orientedsubstantially throughout; that is, the art has frequently preparedcontainers using a tubular extrusion and orientation technique followedby a pinching or like operation to close off and seal at least one endof the tube. Such pinched and sealed oli portion typically has ainvention low strength, e.g. impact strength, relative to the rest ofthe oriented container.

In accordance with this invention, there is prepared a container orlike-shaped article of thermoplastic material using an injection moldingtechnique, said article having relatively balanced axial orientationsubstantially throughout the thermoplastic material including any gateportion thereof which may be formed during the injection molding.

Thermoplastic as used herein is defined as including polyethylene,polypropylene, polystyrene, polyvinyl chloride, and other well knownthermoplastic materials, e.g. as disclosed for example in US. LettersPatent 3,231,642.

In the broad practice hereof, this invention comprises the preparationof a thermoplastic container-shaped article having uniformly controlled,balanced biaxial orientation molding a homogeneous melt of thermoplasticto form a parison, cooling the parison to a temperature range where thethermoplastic material can be biaxially oriented by stretching andinflating without rupture said temperature range being below thehomogeneous melt temperature, and then stretching the cooled parisonalong its longitudinal axis and inflating it along a second axis whichis radial with respect to the longitudinal axis so as to obtain biaxialorientation of the thermoplastic article.

Biaxial orientation as used herein is defined as includin at least someorientation along both the longitudinal and radial axes. Thus it iscontemplated that there may be a varied amount of stretching andorientation along one axis relative to the other axis or there may be anequal amount of stretching and orientation along each axis.

In accordance with a specific embodiment hereof, this invention involvesforming, by injection molding or the like, a parison or blank havingaxial and radial dimensions equal to not more than about one-half,preferably about one-third to one-fifth, of the dimensions of thefinally stretched and inflated article.

As is customary in the injection molding of thermoplastic materials,such injection molding is carried out with the thermoplastic materialbeing at least at its homogeneous melt temperature.

Although the exact homogeneous melt working temperature will be afunction of the particular polymer, pressure, ingredients incorporatedtherein, and other factors, typical homogeneous melt temperatures not byway of limitations are at least about 300 F. for polyethylene and atleast about 340" F. for polyvinyl chloride.

The parison or hollow body is then cooled to a temperature range whereit can be oriented by axially stretching and inflating without ruptureto its final configuration.

The exact temperature range at which the parison is oriented bystretching and inflating will be a function of substantially the samefactors influencing the homogeneous melt temperature.

For polyvinyl chloride, it has been found that suitable results areobtained using a parison mold temperature of about 220 to 250 F.

For some crystalline-containing polymers such as polyethylene, theparison is oriented by stretching and inflating below a temperature atwhich crystallites are no longer detectable.

The longitudinal stretching of the parison initially accomplishesmono-axial orientation of the polymeric molecules, while the radialinflating or blowing of the parison to its final configuration completesthe biaxial orientation. It is of course contemplated that some radialorientation may occur during the stretching and some longitudinalorientation during the inflating. It is also contemplated that thestretching and inflating may be more or less simultaneous.

In accordance with the aforementioned specific embodiment hereof, thedegree of stretching along each axis to effect biaxial orientationranges upwardly from a minimum of about 200% elongation, preferablyabout 300% to 500%, although elongations of as much as 600% or more maybe used. Expressed in another manner, the degree of stretching andinflating can be defined as that sufiicient to reduce the wall thicknessof the parison by a factor of at least four as determined by themultiple of the stretching and inflating ratios; that is, the thicknessreduction is inversely proportional to the product of the amounts ordegrees of stretching and inflating.

In one preferred embodiment hereof it has been discovered that highlyunexpected results can be obtained with thermoplastic materialcontaining a substantial portion of polyvinyl chloride, e.g. at leastabout 85 percent by weight polyvinyl chloride based on the total weightof the material.

In accordance with such embodiment, there is prepared an injection andblow molded biaxially oriented, polyvinyl chloride container having atensile impact strength of at least 130, typically at least 200, footpounds mr square inch (ft.-lbs. per inch using ASTM D-l822 testingprocedure, type S specimen. When such polyvinyl chloride container istested to failure using a bottle drop impact test, e.g. where thecontainer is filled with liquid and dropped from a suitable height suchas disclosed in US. Letters Patent 3,288,317, column 13, lines 3 etseq., there results a high energy, brittle crack propagation which isnot perpendicular to the plane of the container wall as ischaracteristic of a typical brittle failure of prior art containers.

For further understanding of this invention, reference is made to thedrawings and the figures thereon.

The drawings FIGURE 1 is a vertical sectional view, with parts shown inelevation, of an apparatus capable of carrying out the presentinvention, the apparatus being illustrated as positioned for theinjection molding of a parison;

FIGURE 2 is a view similar to FIGURE 1, but illustrating the axialelongation of the parison prior to inflation of the parison; and

FIGURE 3 is a view similar to FIGURES 1 and 2, and illustrating theinflation and radial distention of the stretched parison to form thefinal article.

Before explaining the present invention in detail it is to be understoodthat the invention is not limited in its application to the details ofconstruction and arrangement of parts illustrated in the accompanyingdrawings, since the invention is capable of other embodiments and ofbeing practiced or carried out in various ways. Also, it is to beunderstood that the phraseology or terminology employed herein is forthe purpose of description and not of limitation.

In FIGURE 1, reference numeral refers generally to an apparatus similarto that illustrated and described in detail in US. Letters Patent3,172,929 issued to Thomas R. Santelli.

Generally, the apparatus includes a pair of parison mold sections 11circumscribing the outer surface of a parison mold chamber 12,preferably in the form of a truncated cone. The parison mold is providedwith passages for heat exchange fluid, these passages being indicatedgenerally at 13 and being adapted to circulate hot or cold heat exchangefluid to very accurately control the temperature of the parison moldchamber 12.

The parison mold sections 11 are recessed at their lower ends, as at 14,to receive therein the upper end of an injection cylinder, indicatedgenerally at 15, which is axially bored, as at 16, to receivetherethrough an injection plunger 17 provided with an enlarged head 18defining, when in its elevated position of FIGURE 1, the lower extremityof the mold chamber 12.

Projecting downwardly into the mold chamber 13 is an internal sleeve 20having its lower end tapered, as at 21, to provide a tapered exteriorsurface 22 forming the inner surface of the mold chamber. The taperedportion 21 of the sleeve 20 terminates in an open lower end 23 which isclosed by a valve element 25 having an actuating stem 26 projectingaxially through the sleeve. As illustrated in FIGURE 1 of the drawings,the valve element 25 is in its upper or retracted position abutting theopen lower end 23 of the sleeve 20 and cooperating with the surface 22of the tapered portion 21 of the sleeve 20 to define the lower extremityof the inner wall of the mold space 12. The lower exposed face 27 of thevalve 25 is parallel to and in opposed relation to the free upperextremity of the plunger head 18.

Disposed interiorly of the sleeve tapered portion 21 is a heat exchangecoil 28 in intimate surface contact with the inner surface of thetapered portion 21 of the sleeve 20 so as to control the temperature ofthe sleeve. The heat exchange passages 13 in the parison mold sections11 and the heat exchange coil 28 interiorly of the sleeve portion 21thus very accurately control the temperature of the mold space 13, forreasons which will hereafter become apparent.

Surrounding the sleeve 20 are a pair of separable neck mold sections 30,these sections projecting radially inwardly to terminate in semi-arcuatesurfaces 31 contoured to define, for example, the exterior surface of acontainer neck or the like. The neck mold sections are also providedwith heat exchange fluid passages 32. The inner surfaces of thecontainer neck are defined by a neck ring sleeve 35 closely peripherallysurrounding the cylindrical portion of the sleeve 20 and having 21depending annular flange 36 interposed betwen the neck ring sections 30and the sleeve, so that the smooth periphery 37 of the ring defines theinner surface of the container neck.

As described in the above-identified Santelli patent, a measured chargeof a plasticized thermoplastic material, e.g. polyethylene,polypropylene, polystyrene, or polyvinyl chloride, is deposited in theaxial feed passage 16, and the plunger 18 is displaced verticallyupwardly to move the measured charge of plasticized material into themold space 12. Thus, the shape illustrated in FIGURE 1 is injectionmolded.

Preferably, a relatively cool heat exchange fluid such as water or airis circulated through the heat exchange passages 32 of the neck rings 30to chill the neck portion in surrounding relation to the cylindricalsleeve 20. Thus, the neck portion; ie that portion of the parisonsurrounding the opening in the final container, is injection molded toits final configuration and is firmly anchored in place in the mold.

At the same time, chilled heat exchange fluid is circulated through theinterior coil 28 and the exterior heat exchange passages 13 to controlthe temperature of the remainder of the parison to slightly less thanthe melt temperature of the specific material being utilized. Thetemperature of that portion of the parison which will sub sequently beblow molded is controlled within an orientation range below thehomogeneous melt temperature as described hereinbefore.

After the desired temperature has been attained, the parison is removedfrom the mold preferably by vertically upwardly withdrawing the parison(as described in detail in said Santelli patent), or, alternatively, byopening the parison mold sections, and the valve element 25 (the movableparison core element) is moved relative to the sleeve 20 to thepositions illustrated in FIGURE 2 of the drawings. Such relativemovement stretches along the longitudinal axis those portions of theparison formed intermediate the tapered portion 21 of the sleeve and theparison mold 11, while the highly chilled portions of the materialconstituting the neck of the parison remain in place intermediate thecylindrical portion of the sleeve 20 and the surrounding neck moldsection 30.

The extent of longitudinal or axial stretching of the parison should bein excess of about 200%, preferably about 300% to 500%, althoughstretching of at least about 600% can be accomplished. Thus, the wallthickness of the parison will be reduced by a factor of at least two.This axial stretching initially orients the polymeric molecules in adirection parallel to the direction of the stretching, i.e. parallel tothe axis of movement of the valve element 25.

Subsequently, the parison is blown to its final configuration in a blowmold 40, as illustrated in FIGURE 3 of the drawings, by the injection ofa gaseous medium, eg, a pressurized fluid such as air, through thesleeve 20 and into the parison. The blowing radially inflates theparison so as to orient the polymeric molecules in a directiontransverse to the orientation effected by the axial stretching of FIGURE2. Thus, the sequential axial and radial stretching of the injectionmolded parison etfects biaxial orientation in the final article andgreatly enhanced physical characteristics result.

Several important aspects of the process illustrated in FIGURES 1, 2,and 3 must be emphasized. These are:

l. The stretching in each direction must take place while the injectionmolded parison is at a temperature within a narrow temperature rangedependent upon the material being processed, as described hereinbefore.This requires that the portions of the parison which are to be stretchedmust be at a substantially constant overall temperature. Thus, theinjection molding of the parison becomes critical in order to alford thenecessary thermal control. This degree of thermal control cannot beobtained in a continuous forming operation, as in processes in which anextruded tube is utilized as the parison.

2. The degree of stretching, defined as being greater than 200% andpreferably within the range of from 300% to 500%, is accomplished ineach direction. Thus, an axial elongation of greater than 200% isrequired and a blow up ratio, in order to effect radial stretching, inexcess of 2 to 1 is also required. The effect on wall thickness is atwo-step reduction by a factor of at least two in each direction withtotal wall thickness reduction of four. The parison sidewalls and bottomwalls are of an average wall thickness which is substantially greaterthan the average thickness of the side and bottom walls of the blownarticle. The stretching of the parison both radially and axially reducesthe average wall thickness of the side walls and bottom walls of theblown article.

3. The sequential stretching and inflating steps may be carried outseparately or in combination. Thus, the stretching of FIGURE 2 may becarried out after the parison is enclosed in a blow mold, although thereis no reason why the axial stretching cannot be carried out, at least inpart, during the transfer operation from the parison mold to the blowmold.

From the foregoing description, it will be readily appreciated by thoseskilled in the art that the present invention provides a new and novelmethod of making a plastic article of greatly enhanced physicalcharacteristics.

I claim:

1. A process for preparing a biaxially oriented, hollow,container-shaped, thermoplastic article of substantially increasedstrength comprising the steps of injection molding a homogeneous melt ofa molecularly orientable thermoplastic material to form a parison,cooling the parison to a temperature range at which the thermoplasticmaterial is highly molecularly orientable in a direction in whichstretched, said range being below the homogeneous melt temperature, andthen first stretching the parison along a longitudinal axis at least200% relative to the original length suflicient to reduce the wallthickness of the parison at least 50% relative to the original wallthickness of the parison so as to form an elongated tube having oneclosed end in which the material of the tube is uniaxially molecularlyoriented and, subsequently to completion of the stretching operation,radially expanding the elongated tubc including the closed end byinflation with respect to said longitudinal axis sufficient to increasethe diameter of said stretched elongated tube in excess of 200% relativeto the original diameter such that the wall thickness of the stretchedand expanded parison is 25% or less of the original parison thickness,thereby producing substantially complete biaxial orientation in thematerial of the inflated thermoplastic article.

2. The process of claim 1 wherein the degree of said longitudinalstretching and of said radial expansion is at least 300%.

3. The process of claim 2 wherein the thermoplastic material comprisesat least polyvinyl chloride by weight and said temperature range isbetween 220 and 250 F.

4. In a process of producing a hollow, blown plastic article from amolecularly orientable thermoplastic material by injecting molding ahollow parison having a closed bottom end about a core having an axiallymoveable end element abutting the closed bottom end of the parison andthen blowing the parison, the improvement of cooling the molded parisonto a temperature below the homogeneous melt temperature of thethermoplastic material at which the material is highly orientablewithout rupture in the direction of stretch, maintaining said parison atsaid temperature while (1) axially stretching said parison including theside and bottom wall by axially moving the core element through adistance at least three times the axial extent of the parison whileretaining fixed the other axial extremity of the parison so that thewalls of said parison are stretched to substantially the axial extent ofsaid article and the material uniaxially molecularly oriented andthereafter (2) radially expanding and stretching the side and bottomwalls of the stretched parison by injecting a gaseous medium underpressure into the stretched parison to inflate the parison to atransverse dimension at least three times of that of the stretchedparison and establish substantially complete biaxial molecularorientation of the material in the inflated article.

5. A process for preparing a biaxially oriented, hollow, blown plasticarticle from a molecularly orientable ther moplastic material,comprising the steps of injection molding a homogeneous melt of saidmolecularly orientable thermoplastic material to form a parison havingside walls merging into a closed bottom wall, the average wall thicknessof the side and bottom walls of said parison being substantially greaterthan the average thickness of the side and bottom walls of said article,and stretching the parison both axially and radially to reduce theaverage wall thickness of the side walls and bottom walls of saidparison to the average thickness of the side and bottom walls of saidarticle, the stretching taking place while the parison walls are at atemperature at which the thermoplastic material is highly molecularlyorientable in all directions when stretched and the stretching being atleast partially carried out in a blow mold enclosing the parison anddefining the shape of the final article, thereby biaxially orienting thematerial of the inflated thermoplastic article.

6. In a method of making a hollow, blown plastic article from amolecularly orientable plastic material which is orientable in alldirections in which stretched, the steps of injection molding ahomogeneous melt of said molecularly orientable thermoplastic materialto form a parison having side walls merging into a closed bottom wall,cooling the parison to a temperature range in which the thermoplasticmaterial is highly molecularly orientable in all directions in whichstretched, and then stretching the parison both axially and radially toreduce the average wall thickness of the side walls and bottom walls ofsaid parison to the average thickness of the side walls and bottom wallsof the finished article, said step of stretching 7 8 said parison beingat least partially carried out by inflating 3,172,929 3/1965 Santelli264-97 the parison a! a blow ratio of at least 2 r0 1 in a blow3,244,778 4/1966 Ninneman 26497 X mold defining the shape of the finalarticle. 3,341,644 9/1967 Allen 264-97 References Cited 5 ROBERT F.WHITE, Primary Examiner The following references, cited by the Examiner,are J, H SILBAUGH, Assistant Examiner of record in the patented file ofthis patent or the original patent. US. Cl. X.R.

UNTED STATES PATENTS 425-DIG. 20s, DIG. 209, DIG. 216

2.331.688 10/1943 Hobson 18-5 10 2,878,513 3/1959 Slaughter l819

